the Uniate Church (especially in western Ukraine) at the expense of the often persecuted Orthodox religion, and denying the Ukrainian people cultural opportunities were some of the factors that contributed greatly to the unrest. In spite of the suppression, during the first half of the 17th century the Rus' elements in the region matured, consolidated and gained strength at all levels of society. Accordingly, a great majority of the Ukrainian nobility from the Kiev and Bratslav Voivodeships and the Ukrainian urban people joined the uprising as well. The unusually dry summer of 1648 and locust attacks caused destruction of crops and hunger in Ukraine, adding to the region's instability. As Władysław IV carried out preparations for the Turkish expedition that he had planned, by the time the Sejm halted the undertaking, large numbers of Cossack warriors had been mobilized and put in a state of military readiness. The appearance of an outstanding leader, a scribe in the Zaporizhian Army of Registered Cossacks, Bohdan Khmelnytsky, was another decisive factor. Having suffered a wrong from Daniel Czapliński, a Polish nobleman, and unable to obtain redress through official channels, Khmelnytsky headed for the Zaporizhian Sich, where he agitated among the Cossacks already embittered by the repressions of the 1630s, and now also made restless by the cancellation of the war they were told to expect. Khmelnytsky effectively caused the Cossack force to change sides. He was able to secure support from the Crimean Khanate, taking advantage of the Tatars' (and their Ottoman overlords') interest in disabling the Commonwealth's offensive military capabilities. The Cossack-Tatar alliance, a new factor in the regional civil warfare, turned out to be militarily highly effective. The disastrous consequences of the erroneous and short-sighted, during the reigns of the first two Vasa kings, Cossack policies of the Polish Republic of Nobility were about to make their impact, first of many. In April and May 1648, in the Battle of Zhovti Vody and the Battle of Korsuń the joined Cossack-Tatar forces completely destroyed the Crown army, capturing Crown Hetmans Mikołaj Potocki and Marcin Kalinowski. Thousands of Registered Cossacks, sent into combat with the Polish forces, were persuaded by the rebels to change sides. As a result of the dramatic events, vast expanses of Dnieper Ukraine became engulfed in the spreading social upheaval and the region's separation from the Polish kingdom had begun. Ukrainian land potentate Jeremi Wiśniowiecki, who embarked on the